Currently working on a project for the first time using the Agile method, and the way we've broken down stories into tasks essentially mandates that we work vertically. Its also partly because the first few iterations are designed to cut through the center with the idea that we will have a functional skeleton by Feb next year. This definitely requires serious skill from the devs, as you end up coding so many different parts in parallel with other people. You might have three different devs, for example, coding ways to manage configuration processees and serialization. If you aren't careful, the conceptual integrity of the application will be shot to shit.